OPDS: allow the use of file URIs

[llemeurfr]

In situations where the internet is not available and shared ebooks (especially textbooks) are stored in a network disk drive (i.e. a NAS + Wifi), it is useful to enable the use of OPDS feeds referenced via a file URI, and in which every reference is also a file URI.

On Windows, file paths are formatted as file:///c:/path/to/the%20file.txt.
On a Mac or Linux box, they are formatted as file://hostname/path/to/the%20file.txt, or file:///path/to/the%20file.txt` for localhost.

[danielweck]

I really think we need to discuss the ramifications of enabling local filesystem access. Moving to a discussion.

[danielweck]

See PR: #1549